Quantum interference effects in superconducting rings containing one or two Josephson junctions are well known. Devices exhibiting these effects, known as superconducting quantum interference devices or SQUIDS, have been used, for example, in the measurement of magnetic fields and voltages. Both dc SQUIDS using two Josephson junctions connected in a superconducting ring and RF SQUIDS incorporating a single junction in a superconducting ring have been used for making such measurements. Such dc SQUIDS have recently been constructed using thin film Josephson junctions having oxide or semiconductor barriers. Such dc SQUIDS have been shown to have advantages in regard to magnetic energy sensitivity compared to the RF SQUID. The construction and operation of tunnel junction dc SQUIDS is described in detail in the article "Tunnel Junction dc SQUID: Fabrication, Operation, and Performance" by Clarke, Goubau, and Ketchen, Journal of Low Temperature Physics, Vol. 25, Nos. 1/2, 1976, pp. 99-143.
Recent efforts at obtaining improved energy sensitivity have involved the use of very small area junctions which, because of their relatively low capacitance, make possible higher resistance SQUIDS. Such small area junctions (approximately 1.mu. diameter) are able to operate in the earth's ambient field, thus avoiding the problem of providing a low field environment.
The principal problem with these small area junctions has been their tendency to exhibit a large amount of low frequency noise having a 1/f energy spectral density. This effect has been observed and analyzed as resulting from the thermodynamic temperature fluctuations in the superconducting material on either side of the junctions. The amplitude of these fluctuations is inversely proportional to the volume of material so that the smaller the junction, the greater the amplitude of these fluctuations. These temperature fluctuations produce variations in the effective operating resistance of the junctions, and since a dc bias is normally impressed across the junctions, these resistance changes give rise to fluctuations in the partitioning of the dc bias current between the two junctions. These current fluctuations, in turn, result in fluctuations in the magnetic flux coupled to the SQUID having the same 1/f spectral distribution as the original temperature fluctuations.